Hydraulic machines such as dozers, loaders, excavators, backhoes, motor graders, and other types of heavy equipment use one or more hydraulic actuators to accomplish a variety of tasks. These actuators are fluidly connected to a pump of the machine that provides pressurized fluid to chambers within the actuators, and also connected to a sump of the machine that receives low-pressure fluid discharged from the chambers of the actuators. As the fluid moves through the chambers, the pressure of the fluid acts on hydraulic surfaces of the chambers to affect movement of the actuators. A flow rate of fluid through the actuators corresponds to a velocity of the actuators, while a pressure differential across the actuators corresponds to a force of the actuators.
Control over the speed and/or force of hydraulic actuators can be provided by way of one or more metering valves. For example, a first metering valve controls fluid flow into a head-end of a hydraulic cylinder, while a second metering valve controls fluid flow out of the head-end. Likewise, a third metering valve controls fluid flow into a rod-end of the hydraulic cylinder, while a fourth metering valve controls fluid flow out of the rod-end. The different metering valves are cooperatively opened and closed (e.g., based on operator input) to cause fluid to flow into one end of the hydraulic cylinder and simultaneously out of an opposing end, thereby extending or retracting the hydraulic cylinder.
A conventional metering valve includes a body having a bore that receives a spool, and two or more passages formed in the body that communicate with each other via the spool. The spool is generally cylindrical, and includes lands that extend outward away from the body at either side of a valley or annular groove. When the lands are positioned at one or more entrances of the passages, the spool is in a flow-blocking position. When the spool is moved to a flow-passing position, the valley bridges the entrances such that fluid communication between the passages is established via the valley.
Conventional valves can be massive and require a significant amount of energy to move them quickly between the flow-blocking and flow-passing positions. For this reason, pilot valves are often connected to opposing ends of the spool, and function to selectively communicate pressurized pilot fluid with the spool ends. The pilot fluid functions to urge the spool between positions. Using multiple pilot valves, however, can be expensive and consume a large amount of space in the associated valve block.
One attempt to address the issues discussed above is disclosed in U.S. Pat. No. 6,637,461 (the '461 patent) by Post that issued on Oct. 28, 2003. In particular, the '461 patent discloses a valve assembly having a primary control spool configured to meter fluid flow to a work port, and a piston connected to one end of the control spool to define first and second opposing pressure chambers. First and second electrohydraulic actuators are connected to control movement of the control spool via the piston. The first electrohydraulic actuator is a metering actuator configured to proportionally meter pressurized fluid. The second electrohydraulic actuator is an on/off actuator configured to selectively connect the metered fluid or a tank pressure with the first pressure chamber. The second pressure chamber is always filled with pressurized fluid, but has a smaller hydraulic surface area than the first pressure chamber. Thus, when pressurized fluid is not being directed into the first pressure chamber, the fluid pressure in the second pressure chamber creates a force imbalance on the piston that causes the primary control spool to move in a first direction. And when pressurized fluid is being directed into the first chamber, the fluid pressure in the first chamber creates a force imbalance on the piston that causes the main control spool to move in a second direction. A feedback pin rides on ramped surfaces of the piston and provides force feedback to the first electrohydraulic actuator.
Although the valve assembly of the '461 patent may benefit from having electrohydraulic actuators at only one end of the control spool, it may still have limited application and high cost. In particular, because the valve assembly requires that a separate piston be connected to the primary control spool, the valve may be long. This may prohibit use of the valve in applications that are space-constrained. In addition, because the piston is separate from the primary control spool, the increased component count may increase a cost of the valve and an assembly cost and difficulty.
The disclosed valve is directed to overcoming one or more of the problems set forth above and/or other problems of the prior art.